1. Field of the Invention
The present invention relates to an image forming apparatus.
2. Description of the Related Art
Techniques adopted by image forming apparatuses, such as printers, facsimiles, and copiers, for reducing density variations and correct color misregistration caused by a lapse of time and environmental change include a technique of forming test toner patches on a transfer belt and detecting densities and positions of the patches.
Japanese Patent Application Laid-open No. 2002-207337 discloses such a technique for bringing yellow, magenta, cyan, and black toner images in registration by forming a plurality of toner patches for use in correcting color misregistration on a transfer belt every time the number of output sheets reaches a predetermined number at power-on or the like.
There is also known a similar technique for performing density adjustment on toner-image forming devices, each of which forms a toner image of one color, in a full-color image forming apparatus.
Japanese Patent Application Laid-open No. 2006-293240 discloses a technique for performing density correction by forming toner patches in a non-image frame during a period when a to-be-output image (hereinafter, “output image”) is formed.
A large number of currently-used full-color image forming apparatus adopts such a technique as that disclosed in Japanese Patent Application Laid-open No. 2002-207337 for reducing density variations and correcting misregistration by (i) disabling image-output processing every time the number of output sheets reaches a predetermined number, (ii) forming a plurality of toner patches on a transfer belt, and (iii) detecting densities and positions of the toner patches.
However, the technique described above is disadvantageous in that the process of forming the plurality of toner patches in every corresponding color and detecting the patches is time consuming, and an image cannot be output during the process. As a scheme for resolving this problem, lengthening time intervals between corrective detections is conceivable. However, this scheme can disadvantageously make image density less stable or cause color misregistration to be likely to occur.
The technique disclosed in Japanese Patent Application Laid-open No. 2006-293240 does not involve such a period as that described above during which image output is disabled because the toner patches are formed concurrently with image-output processing. However, because the toner patches are formed outside an image area, the number of patches that can be formed is limited; accordingly, control becomes less accurate. Thus, this technique is not satisfactorily appropriate for full-color image forming apparatuses for which tone stability is of primary importance for the sake of printing photographic images and the like.
Therefore, there is a need for an image forming apparatus capable of stabilizing image densities and reducing color misregistration without downtime.